This invention relates to catalytic combustors, especially those made of a stack of alternating corrugated and flat pieces of metal foil, defining a plurality of channels for gas flow.
It has been known to make catalytic combustors by providing one or more strips of metal foil, stacking and/or folding the strips to form a monolith, and coating all or part of the monolith with catalyst. Examples of such combustors are given in U.S. Pat. Nos. 4,576,800, 5,202,303, and 6,060,173, the disclosures of which are incorporated herein by reference.
Catalytic combustors typically include flat strips alternating with corrugated strips. The corrugations hold the flat strips apart, and thereby prevent the monolith from collapsing. The corrugations also serve to define a cross-section having a large number of channels or cells.
As described, for example, in U.S. Pat. No. 5,202,303, it is advantageous to provide a catalyst coating on fewer than all of the channels of the combustor. The coated channels can be designated xe2x80x9chotxe2x80x9d and the uncoated channels can be designated xe2x80x9ccoldxe2x80x9d. The use of cold channels, interspersed with hot channels, prevents xe2x80x9crunawayxe2x80x9d combustion wherein the temperature of the combustor could become great enough to destroy the catalyst.
One disadvantage of the combustor described above lies in the difficulty of starting combustion. The process of initiating combustion is known as xe2x80x9clight-offxe2x80x9d. Approximately the first inch of the inlet end of the combustor is known as the light-off zone, because it is there that the combustion begins. In a combustor in which coated and uncoated channels alternate with each other throughout the combustor, the inlet temperatures must be unreasonably high to achieve light-off.
One improvement which addresses the above problem is described in U.S. patent application Ser. No. 09/586,482, filed Jun. 1, 2000, entitled xe2x80x9cCatalytic Combustor Having Reduced Light-Off Temperaturexe2x80x9d, the disclosure of which is incorporated by reference herein. In the combustor disclosed in the cited application, there is a band of catalyst coating, provided along a portion of the inlet end of the combustor, on a side of the strip which would otherwise be free of catalyst coating. This extra coated band works well to facilitate light-off because, at the inlet end of the combustor, all of the channels are hot rather than cold.
However, the above approach has some disadvantages. Since there is now some combustion in the channels intended for cooling, the overall catalyzed combustion for the system must be more than 50%, assuming a design in which half the channels are coated and half are uncoated. Moreover, the amount of combustion in the cooling channel is somewhat unpredictable, because the reaction is governed by both kinetics and mass transfer. Modeling and experiments have shown that minor changes in inlet temperature can lead to wild excursions in outlet temperature.
The ideal catalytic combustor is one in which 1) light-off occurs at a relatively low temperature, 2) the increase in outlet temperature occurs very rapidly after light-off, and 3) the outlet temperature quickly stabilizes, at a final operating temperature, shortly after light-off. The present invention provides a combustor which achieves all of these goals.
The present invention comprises a catalytic combustor formed of a plurality of corrugated strips alternating with a plurality of flat strips. The corrugated and flat strips together define a plurality of channels, some of the channels being coated with catalyst and some of the channels being uncoated. The combustor is modified, at the inlet end only, in one or both of the following two ways. First, there may be a thermal barrier, located along the boundary of at least one of the coated channels, for inhibiting the flow of heat from the coated channel to an adjacent uncoated channel. Secondly, there may be an additional coated strip, located within at least one of the coated channels, for enhancing the catalytic combustion that occurs in the coated channel, thereby improving the light-off performance of the combustor.
The above-described modifications, namely the thermal barrier and the additional coated strip, may be present separately or in combination. Also, there may be two or more additional coated strips, disposed within one of more of the coated channels.
The thermal barrier may be an insulating layer, disposed on the boundary of the coated channel, the insulating layer being located between the wall of the channel and the catalyst. The barrier could also be a separate strip or fabric, or other member capable of providing thermal insulation and of holding a catalyst. The thermal barrier may also include an air gap between the member that holds the catalyst and the wall of the channel. The preferred thermal barrier is a thermally insulating coating that is sprayed onto the wall of the channel, such that the catalyst can be added to the insulating coating. The latter technique avoids the need for registration of a strip or fabric with the primary corrugated strip.
The present invention therefore has the primary object of providing a catalytic combustor.
The invention has the further object of improving the light-off characteristics of a catalytic combustor.
The invention has the further object of reducing the temperature of light-off, reducing the time to achieve a stabilized operating temperature, and limiting the final operating temperature, in a catalytic combustor.
The invention has the further object of providing a catalytic combustor which lights off quickly, but in which the operating temperature is controlled so as not to harm the catalyst.
The reader skilled in the art will recognize other objects and advantages of the present invention, from a reading of the following brief description of the drawings, the detailed description of the invention, and the appended claims.